Deflation is a model applied to monetary policy that works by controlling the circulating supply of an asset against expansion.
Traditional monetary models, such as fiat currency, are inflationary. The units that are in circulation are increased every year by ~1-4%. The theory behind inflation is that there is always growing economic demand alongside a growing population. While there is merit behind the theory, it exposes its user base to constant devaluation. Deflation on the other hand, does not allow for supply to grow, and promises a better environment to preserve value. A prime example of a deflationary model is that of gold or Bitcoin; there is a limited supply of both at any given moment.